1. Field of the Invention
The present invention relates to an optical device which separates optical signals with multiple wavelengths transmitted by means of an optical fiber into signals of every wavelength, and in particular, relates to a wavelength separation optical device, also known as an optical wavelength demultiplexer, which separates optical signals for every wavelength by making use of an optical filter and a multiple wavelength light transmission module using the wavelength separation optical device.
2. Conventional Art
JP-A-2000-162466, which corresponds to U.S. Pat. No. 6,198,864B1, discloses an optical demultiplexer in which a plurality of wavelength specifying filters each of which only permits transmission of a specified wavelength component and reflects other wavelength components are arranged, light beams reflected from the respective filters are propagated in a relaying manner by making use of such as an objective mirror and a relay focusing mirror and respective components passed through the filters are detected by photo detectors prepared for every wavelength component as a single component.
However, in the conventional optical demultiplexer, since the optical signals are propagated in a relaying manner by causing reflection on a curved surface of such as the objective mirror and the relay focusing mirror, there arose a problem that such as incident position error and incident angle error of optical signals due to slight displacement of an input use optical fiber are accumulated and enlarged when the same are reflected such as by the objective mirror and the relay focusing mirror, and a positional error of the finally detected optical signals is amplified. For this reason, an attachment position adjustment of the optical fibers was difficult.
Further, it was required to form in an optical block optical parts having a curved surface such as the objective mirror, the relay focusing mirror and a mirror lens array and in order to correctly relay and focus the optical signals the configuration of these optical parts has to be formed accurately, however, since these optical parts are microscopically small, there also arose a problem that an accurate formation thereof is difficult. Still further, it is required to apply a reflection coating on these optical parts, however, such coating has to be applied on a spherical surface, therefore, it was difficult to obtain a high reflectance, even if a dielectric multi layer film is used, thereby, an optical loss is likely to be caused.
The present invention is for resolving the above problems, and an object of the present invention is to provide a wavelength separation optical device of which assembly and adjustment is easy and which shows a small optical loss and further, to provide a multi wavelength light transmission module.
In order to achieve the above object, the present invention provides a wavelength optical separation device including a wavelength separation means which separates optical signals having multiple wavelengths carried through a photo transfer medium for every wavelength and a plurality of photo detectors which receive the optical signals separated by the wavelength separation means, which is characterized by further including a spreading angle reducing means which reduces a spreading angle of the optical signals making incidence from the photo transfer medium and a front polarized light conversion means which converts the optical signals transmitted from the spreading angle reducing means to the wavelength separation means into linear polarized light.
In the wavelength separation optical device of the present invention, by making incidence of the optical signals of linear polarized light aligned substantially in parallel light with the light beam spreading angle reducing means and the polarized light conversion means, the respective wavelength components can be separated and be taken out even when passage of the optical signals is bent by 90xc2x0 by the wavelength separation means. Usually, when a branching angle is large, a difference in characteristic of the wavelength separation means is caused depending on polarized light making incidence thereinto. The difference due to the polarized lights is remarkable at a branching angle of more than 40xc2x0 (an incidence angle of more than 20xc2x0) and, in particular, at a branching angle of more than 60xc2x0 (an incidence angle of more than 30xc2x0). Since, after converting the optical signals into linear polarized light by the polarized light conversion means, the optical signals are made incidence into the wavelength separation filters, a large branching angle can be realized. The polarized light conversion means can be constituted, for example, by a PBS (Polarized Beam Splitter), a xcex/2 plate and a mirror.
For a telecommunication use optical fiber, such as a single mode optical fiber and a multi mode optical fiber have been used. As the multi mode optical fiber, a multi mode optical fiber (50MMF) having core diameter of 50 xcexcm and a multi mode optical fiber (62.5MMF) having core diameter of 62.5 xcexcm are usually used. When these multi mode optical fibers are used, because of the large core diameter a plurality of modes are propagated, therefore, the light beams can not be aligned in complete parallel light beams, even if lenses are used, thereby, a spreading angle is caused. When optical signals having a spreading angle make incidence into the wavelength separation filters, the wavelength separation characteristic varies for every incidence angle. Namely, when an incident angle of a light beam onto a wavelength separation filter increases, the wavelength characteristic thereof shifts toward lower wavelength. For this reason, a spreading angle of incidence light beam is limited depending on the branching angle thereof to suppress the characteristic variation. As the light beam spreading angle reducing means, such as a lens, a refractive index distributing lens and an optical waveguide of which diameter increases in light emitting direction can be used.
Further, the present invention provides a multi wavelength light transmission module including a coupling portion with an optical signal transmission use optical fiber, a wavelength separation optical device, a photo detector array, a signal receiving circuit which amplifies signals from the photo detector array and waveshapes the same, a light source array, a signal transmission circuit which drives the light source array and a wavelength joining device which joins optical signals from the light source array, which is characterized, in that the coupling portion and the wavelength separation optical device are connected by a multi mode optical fiber, the wavelength separation optical device includes a spreading angle reducing means which reduces the spreading angle of the incident optical signals from the multi mode optical fiber and a front polarized light conversion means which converts the incident optical signals from the incident portion into linear polarized light, and the optical signals converted into linear polarized light by the polarized light conversion means are made incidence into the wavelength separation means to separate the same for every wavelength.
Since the wavelength separation optical device of the present invention is provided with the light beam spreading angle reducing means and the polarized light conversion means, the device can perform the wavelength separation even with the multi mode optical fiber. Accordingly, even when either a single mode optical fiber or a multi mode optical fiber is used for the signal transmission use optical fiber, the optical signals transmitted in multiple wavelengths can be separated and detected.